The ability to resolve fine details on retinal images can play a key role in the early diagnosis of vision loss. Certain biochemical and cellular-scale features, which may be present in the early stages of many retinal diseases (e.g., ARMD), cannot be detected today with current funduscopic instruments because of the losses in spatial resolution introduced by the ocular medium of the eye and the lack of selectable spectral data. Additionally, the presence of aberrations within the eye limits the effective input pupil size of a standard fundus retinal imager to about 2 mm. This limit leads to a decrease in the contrast of the small image details due to diffraction effects.
A partial solution to the foregoing problems is to use an adaptive optical system, first for measuring aberrations and then for correcting such aberrations. With such a system, it is possible to increase the system pupil diameter up to 7-8 mm and achieve a resolution on the order of 10 .mu.m. The feasibility of this approach has been demonstrated recently by J. Liang et al., "Supernormal vision and high-resolution retinal imaging, through adaptive optics," J. Opt. Soc. Am. A/Vol. 14, No. 11/November, 1997. They report constructing a fundus retinal imager equipped with adaptive optics that permits the imaging of microscopic structures in living human retinas. The optical system, which is illustrated in FIG. 2 of this reference, includes a deformable mirror for wavefront compensation and a wavefront sensing module including a Hartmann-Shack (also known as a Shack-Hartmann; hereinafter abbreviated "S-H") wavefront sensor. Collectively, the S-H sensor, (which is used to measure the eye's optical aberrations) and the deformable mirror (which is used to make small corrections of the optical aberrations) is sometimes referred to as an adaptive optics system. The deformable mirror is positioned in a plane which is conjugate with both the eye's pupil plane and the front surface of the lenslet array of the S-H wavefront sensor. The S-H wavefront sensor is described in detail in J. Liang, et al., "Objective measurement of wave aberrations of the human eye with the use of a Hartman-Shack wave-front sensor," J. Opt. Soc. Am./Vol. 11, No. 7/July 1994. The displacement of the image, of each of the lenslets in the S-H wave front sensor, on a CCD gives information required to estimate the local wavefront slope. From the array of slopes, the wavefront is reconstructed via a least squares technique into Zernike modes. In operation, a point source produced on the retina by a laser beam is reflected from the retina and received by the lenslet array of the S-H wavefront sensor such that each of the lenslets forms an image of the retinal point source in the focal plane the CCD detector located adjacent to the lenslet array. The output signal from the CCD detector is acquired by a computer, which processes the signal and produces correction signals which, via a feedback loop, are used to control the deformable mirror.
There are a number of limitations associated with the above described instrumentation including:
1. Sensitively to speckle modulation within the eye; PA0 2. The deformable mirror can only provide limited correction; PA0 3. It is a panchromatic instrument, not multispectral; PA0 4. It operates with a limited field of view, on the order of 2-5 degrees; and PA0 5. Several renditions of the S-H output are required to estimate the wavefront.
Further, while it is claimed that it is useful in determining aberrations beyond defocus and astigmatism and providing improved imaging inside of the eye, there is no discussion of its use as a clinical instrument to be used in the diagnosis of the major causes of vision loss and blindness. Finally, the Liang et al. instrument is a laboratory device composed of very expensive one-of-a-kind components.